pgsql: Add mkdtemp() to libpgport.

Add mkdtemp() to libpgport.

This function is pervasive on free software operating systems; import
NetBSD's implementation.  Back-patch to 8.4, like the commit that will
harness it.

Branch
------
REL9_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/a919937f112eb2f548d5f9bd1b3a7298375e6380

Modified Files
--------------
configure                     |    3 +-
configure.in                  |    2 +-
src/include/pg_config.h.in    |    3 +
src/include/pg_config.h.win32 |    3 +
src/include/port.h            |    3 +
src/port/mkdtemp.c            |  293 +++++++++++++++++++++++++++++++++++++++++
src/tools/msvc/Mkvcbuild.pm   |    2 +-
7 files changed, 306 insertions(+), 3 deletions(-)